On September 4, Earth and Neptune are closest for 2017. One day later, on September 5, Neptune reaches opposition, when it is 180 degrees from the sun in our sky. By closest, we don’t mean close. Neptune, the 8th planet outward from the sun, lodges in the outskirts of our solar system and at opposition lies 29 times farther away from Earth than Earth lies from our sun.
Neptune is said to be at opposition – opposite the sun in Earth’s sky – whenever our planet Earth in its orbit passes between the sun and Neptune. That’s what’s happening over the next couple of days. In 2017, the moon turns full only one day after Neptune reaches opposition.
In fact, the full moon will actually occult (swing in front of) Neptune on the night of September 5-6, 2017 (though only in the extreme southern part of the globe). This occultation of Neptune will be nearly impossible to observe, so the International Occultation Timing Association (IOTA) doesn’t bother to list the occultation times as it does for other lunar occultations of Neptune.
Because we’re more or less between Netune and the sun around now, Neptune is rising in the east around the time of sunset, climbing highest up for the night around midnight and setting in the west around sunrise.
As viewed from Earth now, this world is in front of the constellation Aquarius the Water Carrier.
Opposition is a special event. When any planet outside of Earth’s orbit is at or near opposition, Earth comes closest to that planet for the year, and that planet, in turn, shines most brightly in our sky. Even at opposition, however, Neptune, the eighth planet outward from the sun, is not all that close and it’s not all that bright.
In fact, Neptune is the only major solar system planet that’s absolutely not visible to the unaided eye. This world is about five times fainter than the dimmest star that you can see on an inky black night. You’ll need binoculars (at least) and a detailed sky chart to see Neptune in front of the constellation Aquarius.
Even at that, it’ll only look like a faint star. Many sky watchers will find the faint star Lambda Aquarii with the unaided eye and then star-hop to Neptune.
Neptune, the fourth-largest planet, is just a touch smaller than Uranus, the third-largest. You’d have to line up four Earths side by side to equal the diameter of either planet.
Okay, so it’s unlikely you’ll see Neptune unless you have optical aid and a detailed star chart. But there are four bright planets in the September 2017 sky. Look for Jupiter low in the west after sunset and for Venus, Mercury and Mars in the east before sunrise. Saturn is found in the southern sky at nightfall (or as seen from the Southern Hemisphere: high overhead).
Bottom line: We’re closest to Neptune for 2017 on September 4. Neptune’s opposition – when it’s 180 degrees from the sun on the sky’s dome – is one day later, on September 5. You need optical aid to spot it. Links to charts here.